Former FBI boss James Comey was fired by the President over a "lack of discipline", US attorney general Jeff Sessions has claimed.

Speaking under oath in front of the Senate Intelligence Committee, Mr Sessions insisted the former FBI director's investigation into ties between Russia and the Trump campaign played no part in his dismissal.

He said the intelligence agency needed a "fresh start" as it had "problems", adding that Mr Comey had caused "controversy on both sides of the aisle".

Citing one example, Mr Sessions said it was "stunning" that the ex-FBI chief "usurped" the Department of Justice's authority by announcing that Hillary Clinton would not be prosecuted over her emails.

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In his testimony last week, Mr Comey accused the White House of lying about him and the FBI and repeatedly said he believed he was fired because of the agency's investigation into Trump-Russia ties.

Mr Sessions told committee members he could not speak about his discussions with the President concerning Mr Comey, as they were covered under executive privilege.

After admitting Mr Trump had not invoked privilege, Mr Sessions was accused of "stonewalling" by Democratic senator Ron Wyden.

Asked if he colluded with the Kremlin during the presidential campaign, Mr Sessions described claims against him as an "appalling and detestable lie".

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He said "not one thing that was improper" occurred during two meetings he had with Russian ambassador Sergey Kislyak, as he denied speaking with Mr Kislyak on a third occasion.

In March, Mr Sessions removed himself from any inquiry into alleged Russian interference in the elections.

Asked to explain his decision, the attorney general said he believed he had to recuse himself under Department of Justice rules.

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He said: "I recused myself not because of asserting wrongdoing or any belief that I may have been involved in wrongdoing in the campaign."

Hitting out at "scurrilous and false accusations" over his decision, Mr Sessions said he had "dedicated myself to the highest standards".

He said: "The public demands and expects a transparent government and that is what we are giving them.

"These events have only strengthened my resolve to carry out my duty."

The attorney general went on to deny there were "issues" with his recusal that were "problematic".

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Raising his voice, he said: "This is a secret innuendo being leaked there about me and I don't appreciate it."

Democratic senator Martin Heinrich accused Mr Sessions of "impeding" the committee's probe into allegations of Russian meddling in last year's White House race.

He said the attorney general's refusal to speak about discussions with Mr Trump "speaks volumes".